This invention relates to a method for improving protective coating compositions. More particularly, it relates to a method for improving silicone resin coating compositions adapted to be applied to a substrate, and thereby forming a protective abrasion-resistant coating thereon.
Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses, such as for eyeglasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and ordinary weathering. Continuous scratching and marring results in impaired visibility and poor aesthetics, and oftentimes requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimensional stability, as well as being self-extinguishing, and is easily fabricated.
Attempts have been made to improve the abrasion resistance of transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225, 3,986,997, 3,976,497 and 4,177,315, for example, describe such compositions.
Copending U.S. application entitled "Silicone Resin Coating Composition," by Howard A. Vaughn, Ser. No. 964,910, filed Nov. 30, 1978, discloses another abrasion-resistant coating composition. Copending Frye Application, Ser. No. 964,911, filed Nov. 30, 1978, describes the use of small amounts of polysiloxane polyether copolymers to promote coating formation. Copending Frye Application, Ser. No. 91,716, filed Nov. 6, 1978, describes the use of small amounts of ultraviolet screens to improve adhesion on weathering. Copending Kray application, Ser. No. 156,268, filed June 3, 1980, describes superior compositions for priming plastic substrates prior to top coating with the abrasion-resistant compositions; these generally comprise acrylic esters dissolved in a solvent which is somewhat aggressive to the plastic substrate. Somewhat aggressive in this sense means able to soften, but not dissolve. Copending Frye application, Ser. No. 34,164, filed Apr. 27, 1979, describes abrasion resistant coatings especially adapted to coating unprimed acrylic plastic substrates; in these, the alcohol in the composition is replaced with a more aggressive solvent, such as 2-ethoxyethyl acetate or a mixture thereof with 2-butanone. Copending Conroy application Ser. No. 107,994, filed July 18, 1980 discloses that diacetone alcohol, which also has aggressive characteristics of the type mentioned above, can be added to the abrasion resistant compositions to lower their curing temperature below 100.degree. C. The foregoing patents and applications are incorporated herein by reference.
It has now been surprisingly discovered herein that the addition of a small amount of an aggressive solvent to the uv stabilized, polysiloxane polyether-containing coating compositions disclosed in the above mentioned Frye application Ser. No. 91,716 provides very important advantages.
Specifically, these prior compositions have a shelf life at room temperature of about one month. After this period, the initial adhesion to polycarbonate panels primed with a solution of the thermoplastic polyacrylate of the type described in the Kray application becomes variable and the resistance to weathering decreases. If, however, according to the present invention a small amount of a solvent, such as diacetone alcohol, 2,4-pentanedione, cyclohexanone, or ethoxyethyl acetate is added to the coating composition, adhesion will be maintained, as well as abrasion resistance and accelerated weathering resistance. It is significant that this simple step increases the useful life of the coating composition by two- or three-fold and this facilitates its handling (permitting longer transit times) and making diptanks more feasible (longer pot life). The step of this invention also has a decidedly unexpected rejuvenating effect on older compositions which, until now, lose their initial adhesion after standing. Adding the specific, aggressive solvents to such aged compositions restores them to their original state.